This invention relates to devices, including cardiac pacemakers, tachycardia reversion devices, and defibrillators, for measuring the response of the heart to an electrical stimulation very soon after the generation of a stimulus, even when the same electrode is used for stimulating and sensing. More specifically, this invention relates to such devices which are optimized to function properly in noise environments and with suboptimal leads. Furthermore, this invention relates to such devices capable of performing self-diagnoses for determining when accurate sensing is not possible.
It is desirable to accurately measure the response of the heart to an electrical stimulation pulse for a number of purposes. The initial objective for performing such measurements was the development of threshold tracking systems requiring the simple detection and distinction of a systolic event from a subthreshold non-event. More recently, stimulated response analysis has been used to control pacing rate, to detect physiological effects of drugs, and to diagnose abnormal conditions of the heart.
Stimulus polarization artifacts can interfere with the recording and analysis of the stimulated response. Consequently, there arose a need for accurate methods for eliminating the stimulation artifact and identifying the true nature of the stimulated response. The ability to automatically reduce stimulus polarization artifacts is necessary in a system which analyzes the depolarization waveform stimulated by a pulse because, in addition to generating a cardiac response, an electrical stimulus gives rise to a form of noise called the stimulus artifact. When a device generates an electrical stimulus within the heart, it creates electrical charges which are stored in the body tissues. The stimulus polarization artifact is the signal arising from the dissipation of these stored charges. The amplitude of the stimulus polarization artifact is normally so much greater than that of signals arising from a natural heartbeat or the stimulated response that it is usually futile to sense these diagnostic signals until the stimulus polarization artifact charges dissipate. This is especially true when, as in the case of the preferred embodiment of the present invention, the device uses the same electrode for stimulating and sensing.
To rapidly dissipate these charges and minimize the stimulus polarization artifact at the pacing electrode, the device generates stimulating pulses using a technique known as charge balancing. The procedure and circuit for performing charge balancing is disclosed in U.S. Pat. No. 4,821,724, entitled "Pacing Pulse Compensation", which issued on Apr. 18, 1989, and refers to the method as active recharge. This patent is assigned to the assignee of the present application and its disclosure is incorporated herein by reference. In this procedure, the device generates a triphasic stimulus, with the first and third phases being of one polarity and the second being of the opposite polarity. The amplitudes of the first and second phases are substantially proportional to each other. The third phase drives a current through the stimulating electrode until the voltage equals the starting quiescent voltage.
The charge balancing technique, as performed by the preferred embodiment of the present invention, requires circuitry for sensing cardiac electrical activity including natural polarizations, stimulated potentials and artifacts. This sensing circuitry is disclosed in U.S. Pat. No. 4,692,719, entitled "Combined Pacemaker Delta Modulator and Bandpass Filter", which issued on Sep. 8, 1987. This patent is also assigned to the assignee of the present application and its disclosure is incorporated herein by reference.
Stimulating the heart using the triphasic stimulus waveform allows the device to effectively reduce the polarization artifact, but the best balance of the three phases of the stimulation waveform is not predictable. The apparatus and method of the present invention provides a mechanism for automatically adjusting or balancing the triphasic stimulus waveform in vivo.